[MIPS GlobalISel] Improve reg bank handling in MipsInstructionSelector

Introduce helper methods and refactor pieces of code related to
register banks in MipsInstructionSelector.
Add a few detailed asserts in order to get a better overview
of LLT, register bank combinations that are supported at the moment
and reduce need to look at other files.

Differential Revision: https://reviews.llvm.org/D69663